The present invention relates in general to a multichannel amplifier wherein each channel employs an operational amplifier connected to a field-effect transistor to vary the amplifier gain, and more specifically, to a gain-matched multichannel amplifier wherein each channel has a respective DC bias voltage which compensates for differences in the field-effect transistors.
Amplification of electrical signals by a desired gain can be achieved using operational amplifier (op amp) integrated circuits. By providing negative feedback between the output of the op amp and its inverting input, the gain provided by such an amplifier is controlled by external resistances connected to the op amp as is known in the art. If one or more of the external resistances are variable resistances, the amplifier gain may be varied. Thus, a variable-gain amplifier can be realized by employing a potentiometer in the feedback path between the output and inverting input of the op amp, for example.
It is also known to employ a voltage-controlled attenuator such as a junction field-effect transistor (JFET) operating in its linear or triode region of operation as a variable gain-setting resistance used in conjunction with an op amp. Thus, a gate control voltage supplied to the JFET determines the amplifier gain. However, several disadvantages are associated with certain inherent properties of JFETs. Due to uncontrollable variations in the fabrication process of JFETs, various electrical properties, such as pinch-off voltage V.sub.p, on-resistance R.sub.ds(on), and the resistance curve between R.sub.ds(on) and V.sub.p, have a wide variation from one JFET to another JFET of the same design. The use of a JFET as a voltage-controlled attenuator is strongly influenced by these uncontrolled characteristics, thus limiting the usefulness of JFETs as attenuators. Testing of JFETs after manufacture to sort them according to similar characteristics greatly increase the costs associated with using JFETs as altenuators. Furthermore, sorting cannot eliminate all variability in the JFET characteristics.
Variations in the electrical properties of JFETs create problems if JFET-controlled op amps are to be used in multichannel audio amplifiers (e.g., audio limiters) which require that separate channel gains closely track each other. Even sorted JFETs do not provide sufficiently matched characteristics to obtain the required gain tracking.
The co-pending application mentioned above describes an invention that allows the setting of minimum and maximum gain by use of fixed resistors independently of the JFET characteristics. However, with JFET's having different pinch-off voltages V.sub.p, gains may still not track sufficiently well (e.g., in a limiter, one channel may be attenuating while another is not).